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Abstract

Introduction

To evaluate the efficacy of probiotics in preventing nosocomial pneumonia in critically
ill patients.

Methods

We searched PubMed, EMBASE, and the Web of Science for relevant studies. Two reviewers
extracted data and reviewed the quality of the studies independently. The primary
outcome was the incidence of nosocomial pneumonia. Study-level data were pooled using
a random-effects model when I2 was > 50% or a fixed-effects model when I2 was < 50%.

Conclusions

The use of probiotics was associated with a statistically significant reduction in
the incidence of nosocomial pneumonia in critically ill patients. However, large,
well-designed, randomized, multi-center trials are needed to confirm any effects of
probiotics clinical endpoints such as mortality and length of ICU and hospital stay.

Introduction

Nosocomial pneumonia (NP) is a common complication in critically ill patients, particularly
in patients who are intubated for more than 48 hours, and NP is responsible for significant
in-hospital morbidity and mortality [1-3]. When mechanically ventilated patients develop NP, it is known as ventilator-associated
pneumonia (VAP) [1,2]. Multiple hospital-associated risk factors for NP have been identified. These risk
factors are thought to contribute to increased bacterial colonization of the aerodigestive
tract and facilitate the entry of pathogenic bacteria into the lower respiratory tract
[4].

Considerable efforts have been made to evaluate methods for reducing NP. For example,
selective digestive tract decontamination in critically ill patients has been shown
to reduce the occurrence of NP; however such decontamination has also been associated
with increased rates of antimicrobial resistance [5,6]. Several experimental and clinical studies have suggested a promising effect of probiotics
on preventing NP in critically ill patients [7-10].

Probiotics are commercially available microorganisms that when ingested as individual
strains or in combination may offer potential health benefits to the host [11]. Prebiotics are non-digestible sugars that selectively stimulate the growth of certain
bacteria colonies. The combination of pre- and probiotics has been designated as synbiotics.
It is hypothesized that probiotics could potentially reduce the incidence of NP in
critically ill patients through various local and systemic effects that minimize colonization
by more virulent species or optimize host immune defenses. These effects include reducing
overgrowth of potentially pathogenic microorganisms, enhancing gut barrier function,
reducing bacterial translocation, and up-regulation of immune functions [12-19].

To date, clinical research concerning the effects of probiotics in critically ill
patients have provided conflicting results, with some suggesting clinical benefit
[20-29], and others showing no benefit [30-33]. More recently, one study showed that probiotics therapy led to a significant reduction
in VAP rates among treated patients [22]. However, another study showed that daily prophylactic administration of probiotics
was not effective for critically ill patients, notably for those with non-severe sepsis
[30].

Therefore, we performed a systematic literature review and meta-analysis to investigate
the effects of probiotics in critically ill patients using incidence of NP as the
primary outcome, and mortality, length of stay in the ICU and in hospital, and adverse
outcomes as secondary outcomes.

Materials and methods

Data sources and search strategy

To identify studies for inclusion in this review, two authors independently searched
PubMed, the Cochrane Central Database of Controlled Trials, and EMBASE for relevant
studies published up to January 2012. The search was limited to studies conducted
in humans. No language restriction was imposed. Search terms were individualized for
each database. Search terms used included: ['pneumonia' OR 'critically ill' OR 'intensive
care' OR 'trauma' OR 'pancreatitis' OR 'surgical patients'] AND ['probiotics' OR 'prebiotics'
OR 'synbiotics' OR 'lactobacillus' OR 'bifidobacterium']. We also searched the proceedings of major relevant conferences, trial databases,
the reference lists of identified trials, and major reviews.

Study selection

Two reviewers (KXL and YGZ) independently screened studies for inclusion, retrieved
potentially relevant studies, and determined study eligibility. Any discrepancies
were resolved by consensus. Analysis was restricted to double-blind, randomized controlled
trials (RCTs). For this meta-analysis, we considered those RCTs that compared administration
of probiotics vs. placebo in critically ill patients (such as those admitted to an
ICU or having recently undergone abdominal or another major surgical procedure), and
that reported the incidence of NP or VAP. Probiotics could be administered either
alone or in combination with prebiotics.

Data extraction

Two authors independently extracted data from all of the enrolled studies. Extracted
data included study design (for example, year conducted, sample size), patient characteristics,
study methodology (for example, eligibility criteria, method of randomization and
blinding), intervention (for example, type of probiotic agent, dose, route of its
administration and duration), and clinical outcomes. The primary outcome was the incidence
of NP. We used the authors' definitions for NP if they included clinical and radiological
criteria. Secondary outcomes were mortality, length of stay in ICU and in hospital,
and reports of adverse outcomes.

Quality assessment

We formally assessed the methodological quality of each trial using the Jadad score
[34], which incorporates randomization, blinding, and attrition to derive a score of 0
to 5; higher scores indicate higher quality. Two reviewers (KXL and YGZ) independently
appraised the quality of the included trials. Studies were considered to be of low
quality if the Jadad score was ≤ 2 and high quality if the score was ≥ 3.

Statistical analysis

The meta-analysis was performed using Review Manager 5.0 (Cochrane Collaboration,
Oxford, UK). We computed pooled odds ratios (ORs) and 95% confidence intervals (CIs)
from the adjusted ORs and 95% CIs reported in the observational studies. We used Cochrane
Q and I2 statistics to assess the heterogeneity of study results. We predefined heterogeneity
as low, moderate or high with I2 values above 25%, 50%, and 75%, respectively. In the analysis of heterogeneity, we
considered a P-value < 0.10 statistically significant. Study-level data were pooled using a random-effects
model when I2 was > 50% or a fixed-effects model when I2 was < 50%. Publication bias was assessed by a funnel plot using the occurrence of
NP as an endpoint.

Results

Our search retrieved a total of 131 references. After applying the inclusion criteria,
twelve studies were included in this meta-analysis [22-33]. A flowchart for the studies evaluated and the reasons for exclusion are shown in
Figure 1.

Figure 1.Flow chart of study selection. Pooled ORs were calculated using the Mantel-Haenszel (M-H)Estimator. Study-level
data were pooled using a random-effects model when I2 was > 50% or a fixed-effects model when I2 was < 50%.

Study characteristics

Characteristics of the included studies are summarized in Table 1. A total of 1,546 critically ill patients were included in these studies. All studies
were published from 2002 to 2011. Trials were conducted in a diverse array of countries.
Most of the trials were carried out at a single center. Four studies recruited patients
in general ICUs [22,30-32], eight studies recruited patients in surgical ICUs [23-29,33], one study recruited patients who were scheduled for liver transplantation [26], and one study recruited patients with severe acute pancreatitis [33]. Seven of the twelve clinical trials enrolled patients who required mechanical ventilation
(MV) from medical and surgical ICUs [22,23,28-32]. The frequency of probiotic administration ranged from once to twice a day. In the
majority of eligible trials, probiotics were administered via nasogastric or orogastric
tube [22,23,28-32] until the patient's discharge from the ICU or death. In some surgical patients, probiotics
were administered via nasojejunal tubes [24-27,33]. The average Jadad score of these studies was 3.5 (range 2.0 to 5.0) (Table 2). Results of the meta-analyses that explored the effects of probiotics on clinical
outcomes are shown in Table 3.

Table 2. Quality of the twelve studies as assessed by the Jadad score [34]

Table 3. Outcome data of the randomized controlled trials included in the meta-analysis (comparison
of probiotics versus control)

Nosocomial pneumonia and subgroup analyses

Results from twelve trials (1,546 patients) were available to examine the effects
of oral probiotics on the incidence of NP. A low level of heterogeneity was found
among the identified comparisons (I2 = 46%, P = 0.04). Pooled analysis showed that the use of probiotics was associated with a statistically
significant reduction in the incidence of NP in critically ill patients (OR = 0.75,
95% CI 0.57 to 0.97, P = 0.03) (Figure 2).

Figure 2.Forest plot showing the effect of probiotics on the occurrence of nosocomial pneumonia
(NP) in critical ill patients. Pooled ORs were calculated using the Mantel-Haenszel (M-H)Estimator. Study-level
data were pooled using a random-effects model when I2 was > 50% or a fixed-effects model when I2 was < 50%.

Overall mortality

Results of nine trials were available for the analysis of mortality during the entire
hospital stay [22-27,29,32,33]. A meta-analysis of these trials found that probiotics administration had no effect
on overall mortality during the hospital stay (OR = 0.93, 95% CI 0.50 to 1.74, P = 0.82) (Figure 3). We did find evidence of statistical heterogeneity for in-hospital mortality (I2 = 51%, P = 0.07). Only three of the twelve selected RCTs provided information regarding mortality
during an ICU stay [28,30,32]. There was no significant difference in ICU mortality between a probiotics group
and a placebo group (OR = 0.84, 95% CI 0.55 to 1.29, P = 0.43) (Figure 4). There was no heterogeneity between trials (I2 = 0%).

Figure 3.Forest plot showing the effect of probiotics on in-hospital mortality. Pooled ORs were calculated using the Mantel-Haenszel (M-H)Estimator. Study-level
data were pooled using a random-effects model when I2 was > 50% or a fixed-effects model when I2 was < 50%.

Figure 4.Forest plot showing the effect of probiotics on ICU mortality. Pooled ORs were calculated using the Mantel-Haenszel (M-H)Estimator. Study-level
data were pooled using a random-effects model when I2 was > 50% or a fixed-effects model when I2 was < 50%.

Duration of stay in the hospital

Eight studies were included in the analysis of the length of stay in hospital [22,24-27,30,32,33]. There was no apparent effect of probiotics therapy on the duration of stay in hospital,
with a mean difference (MD) of -0.13 days (95% CI -0.93 to 0.67, P = 0.75) (Figure 5). A low level of heterogeneity was found among these comparisons (I2 = 46%, P = 0.07).

Duration of stay in the intensive care unit

Data from eight studies were included in the analysis of the duration of stay in the
intensive care unit [22,24,26,27,29,30,32,33]. There was significant heterogeneity in length of ICU stays (I2 = 68%, P = 0.002) (Figure 6). There was no significant difference between the compared groups regarding this
outcome (MD in days = -0.72, 95% CI -1.73 to 0.29, P = 0.16).

Adverse events

The following adverse events were reported: diarrhea, abdominal cramps and bowel ischemia
[22,25-27,32,33]. Data regarding the incidence of diarrhea were provided in six of the twelve included
RCTs [22,25-27,32,33]. There was no difference between probiotics and placebo groups in the incidence of
diarrhea (OR= 0.85, 95% CI 0.58 to 1.26, P = 0.43, I2 = 0%). There was no significant difference between probiotics and placebo groups in
the incidence of abdominal cramps in the meta-analysis that included only three RCTs
(OR = 0.74, 95% CI 0.47 to 1.17, P = 0.19, I2 = 0%) [25,26,33].

Publication bias

Upon visual inspection of the funnel plot for the primary outcome, we found evidence
of publication bias (absence of small studies, shown in the right lower corner of
Figure 7).

Discussion

Our meta-analysis found that probiotics administration was associated with statistically
significant reduction in the incidence of NP in critically ill patients. However,
the pooled results showed that probiotics did not affect overall mortality, or length
of stay in the hospital and the ICU, which were the secondary endpoints of the study.

The current meta-analysis is different from previous reviews in several aspects. Although
three recent reviews addressed somewhat similar questions, our meta-analysis identified
and included more eligible studies than the previous reviews [35-37]. These previous reviews on probiotics administration included studies that recruited
patients requiring MV only [35] or studies of surgical patients only [36]. Thus, those meta-analyses were limited to selected populations. Trials of any type
of critically ill patients were eligible for our study, and therefore our results
are applicable across a wide range of clinical situations that are encountered with
critically ill patients. In order to diminish the number of confounding factors, we
excluded two studies using chlorhexidine and antibiotic decontamination as control
groups, because the use of chlorhexidine in oral care procedures and antibiotic decontamination
of the digestive tract were considered to be effective in preventing NP [6,21]. The review by Siempos et al. used the data from one trial on the rate of respiratory tract infection as the rate
of VAP [35], which may have contributed to an overestimation of the VAP rate and a greater observed
treatment effect. This trial has been recently published in a separate study [23] and confirms that there are fewer patients with VAP than with respiratory tract infections.
Accordingly, we included the latter study in our meta-analysis. Our results appear
similar to the previous reviews by Siempos et al. [35] and Pitsouni et al. [36], but inconsistent with the results of the systematic review by Watkinson et al. [37]. Siempos et al. found that administration of probiotics was beneficial in the incidence of both
VAP and NP, length of stay in the ICU and colonization rates of Pseudomonas aeruginosa in the respiratory tract [35]. Similarly, the meta-analysis by Pitsouni et al. demonstrated that probiotics significantly reduced the occurrence of postoperative
pneumonia and any infectious complications, as well as the duration of postoperative
hospital stay [36]. While we restricted our subgroup analysis to patients in surgical populations, a
pooled analysis showed a marginally non-significant reduction in NP in favor of probiotics.
In contrast, Watkinson et al. pooled eight trials and found that pre- pro- or synbiotics were not associated with
any significant change in the outcomes studied, that is, length of ICU stay, hospital
mortality and the incidence of nosocomial infection and more specifically incidence
of pneumonia [37]. Although there was no statistically significant effect on the incidence of NP in
all subgroups, the risk reduction associated with probiotics use was substantial.
The reasons for these inconsistent results may partly be due to differences in focus
on clinical outcomes.

The results of this meta-analysis should be interpreted carefully based on other considerations.
As the diagnosis of pneumonia is a more subjective outcome than mortality or length
of stay in the ICU, it may be more subject to bias, and this may in part explain the
marked reduction in pneumonia found in these studies. In addition, the definitions
of pneumonia varied among different studies, which will affect the true nature of
clinical outcomes. In addition, the absence of an effect on secondary outcomes may
be from the small number of pooled RCTs and total patients. And lastly, the treatment
durations in some studies were likely too short to demonstrate maximal benefits. Consequently,
a lack of standard protocols and insufficient numbers of patients may make it difficult
to derive conclusive results based on the current meta-analysis.

The potential harm due to probiotics therapy also warrants comment. The numbers of
patients with diarrhea or abdominal cramps did not differ between those patients who
received probiotics and those who did not in our current meta-analysis. However, a
particular concern in critically ill patients is whether their exposure to probiotics
places them at risk for developing an invasive infection. There were no reports of
bacteremia or sepsis due to probiotics in the studies included in our meta-analysis.
In addition, Besselink et al. found an increased rate of bowel ischemia and mortality in those patients treated
with probiotics [33]. However, a meta-analysis of four RCTs that included severe acute pancreatitis, including
the study by Besselink et al. [38] demonstrated that probiotics did not significantly influence mortality either favorably
or adversely. Accordingly, we should monitor the safety of probiotics as our research
efforts move forward.

Our analysis has several limitations. First, as already mentioned, there was heterogeneity
in the inclusion criteria, the populations studied, the probiotic agents used, doses,
time points when therapy was initiated, durations of therapy, the routes of administration,
and the diagnostic criteria used for establishing NP or VAP. These factors were not
comparable in most of the trials and might have affected the clinical outcomes. These
differences may explain the statistical heterogeneity in some of the secondary outcomes
investigated. Second, even though we were able to pool results across all trials,
the number of patients included in this meta-analysis may not be sufficient to exclude
significant clinical benefit. Third, most trials were done in single centers and may
have had inherent bias related to local practice habits and the populations served.
Although we extensively searched for relevant studies using multiple databases and
multiple search items, and no language restriction was placed on the search, a funnel
plot suggested the possibility of publication bias. Finally, the quality of the included
studies was not consistent. Some RCTs included in our analysis had major methodological
flaws [24,28]. The quality of trials can affect the direction and magnitude of treatment effects
when performing a meta-analysis.

Although the results have been encouraging, there is insufficient evidence to suggest
to clinicians that administration of probiotics is associated with significant clinical
benefit in critically ill patients. In addition, there is a lack of head-to-head comparative
trials with different probiotics to investigate and clarify which probiotic bacterial
strains were of most benefit to critically ill patients. Data regarding the superiority
of different doses of probiotics and routes of administration are also lacking. The
questions that remain to be evaluated in large-scale, randomized controlled trials
of probiotics use in NP include the optimal type of probiotic preparation, administration
route, dose intensity, timing and duration of administration, safety, patient eligibility,
and contraindications.

Conclusions

The use of probiotics was associated with statistically significant reduction in the
incidence of NP in critically ill patients. However, there is no evidence to support
or refute claims of beneficial effects on clinically important outcomes. Large, well-designed,
randomized, multi-center trials are needed to confirm the effects of probiotics in
diverse populations of critically ill patients.

Key messages

• Numerous studies have examined the utility of probiotic therapy to prevent NP in
critically ill patients, but studies on the use of probiotics have yielded mixed results.

• Probiotics administration was associated with statistically significant reduction
in the incidence of NP in critically ill patients.

• More randomized control trials are needed to definitively determine the effect of
probiotics in critically ill patients.

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

KXL and YGZ carried out the primary study search, extracted data, performed statistical
analysis, and drafted and revised the manuscript. JZ drafted and revised the manuscript.
LLT carried out statistical analysis and revised the manuscript. JWL revised the manuscript
and modified the written English. XDW carried out statistical analysis and helped
draft the manuscript. JMQ conceived the idea, participated in its design, and drafted
and revised the manuscript. All authors read and approved the final manuscript.

Acknowledgements

This research was sponsored by the National Basic Research Program (973 Program) in
China (2007CB513004), the Shanghai Subject Chief Scientist Program (07XD14012) and
Shanghai Leading Talent Projects (No. 036, 2010).

References

American Thoracic Society and Infections Diseases Society of America: Guidelines for the management of adults with hospital-acquired, ventilator-associated
pneumonia, and healthcare associated pneumonia.

Joint FAO/WHO working group In Guidelines for the evaluation of probiotics in food: report of a joint FAO/WHO working
group on drafting guidelines for the evaluation of probiotics in food. London, Ontario, Canada; 2002.